Transducer



1958 I w. T. HARRIS 2,834,952

TRANSDUCER Filed March 19, 1955 WWW u u n uuununnfi unnn 22 INVENTOR.

8 WILBUR T HARRIS ub/Jar ATTORNEY 2,831,952? Patented May 13, 1958 Theinvention described herein may be manufactured and used by or for theGovernment of the United States of America for governmental purposeswithout the payment of any royalties thereon or therefor.

This invention relates to transducers and more particularly toelectromechanical transducers for use in either water or air and for awide variety of frequency ranges and preferably having a sensitiveelement of electrostrictive material.

ln the prior art electromechanical transducers embodied piezoelectriccrystal or magnetostrictive sensitive elements in forms which did noallow for matching the mechanical impedance of the transducer to thesound medium in which the transducer was used. Furthermore thetransducers did not have high efficiency over. broad frequency ranges.Crystals were usually employedin theform of blocks suitably assembled inthe transducer structrure. In magnetostrictive transducers in the form'of wound cylinders, rod, laminated rings, or the likeofmagnetostrictive materials were employed in suitable assemblies. Ingeneral, mechanical and electrical considerations dominated transducerdesigns so that manydesirable features could not be convenientlyincorporated:

In order to obtain high eificiency in a' transducer, itis necessary thatit be mechanically resonant; This mechanical resonance in prior arttransducers occurs normally at one discrete frequency, so that highlyefficient transducers normally possess this high efiiciency in a verynarrow frequency range. At other frequencies, their efficiencies: areare low. It has been generally considered impossible to attain highefliciencies in broadband transducers.

The'conical transducer structure of this invention-provides moreeflicient broadband performance in' two 1 important ways. In the firstplace it makes it" possible for the mechanical impedance to be matchedto the sound medium and in the second place the structure hasinherent'in it a distributed resonant eifect for efficient broadband response.

An object of this invention is to provide an electromechanicaltransducer which can be used efliciently in either water or air.

A further object is to provide an electromechanical transducer which canbe used efiiciently in either air or water for a wide variety offrequency ranges.

A further object is to provide an electromechanical transducer in whichthe mechanical impedance can be controlled as desired by choosingdimensions favorably, while still maintaining other advantages.

A further object is to provide an electromechanical transducer ofgeneral applicability, using a very simple and economically manufacturedtype of basic sensitive element.

A further object is to provide an electromechanical transducer elementwhich can be easily incorporated into a directional array assembly,possessing good directional characteristics.

A further object is to provide a basic electromechanical transducerwhich is more versatile as to medium, frequency, range, and manufacture,while also possessing Fig. 2' is an outline showing of anomnidirectional transducer including the basic structure of the deviceshown in Fig. 1,

Fig. 3 is an outline showing of the transducer of Fig. 1 and includingparallel dividing'planes shown in phantom lines,

Fig. 4 is an outline showing a modification of the transducer of Fig. 1,

Fig. 5 is an outline showing of an omnidirectional transducer,

Figs. 6 and 7 are two further modifications of the transducer shown inFig. 1, and

Fig. 8' is afurther modification of a transducer shown inFig. 1.

In Fig. 1 there is shown a transducer 11 adaptable for underwater use.The sensitive element 12' is a hollow cone of electrostrictive material,or more precisely, a piezoelectric ceramic such as a barium titanatecomposition, the conical outer surface 13 of which is coated with a thinmetal electrode 14 to which cable lead 15 is attached and the conicalinner surface 17 of which is coated with a thin metal'electrode18 and towhich cable lead 19'is attached. In order to sensitize hollow cone 12, ahigh voltage is applied between these electrodes 14, 18 to producevolume electrical polarization of the material. After such polarization,the application of pressure to the exterior of the cone 12 will causeradial expansion of the cone thus causing a potential difference toappear across the electrode. The cone 12 is supported on a metal ring21' afi'ixed to-a suitable base 22. The metal ring support is providedwith slots 24 parallel to its axis to provide radial flexibility of thesupport. The interior of the cone 12 is further provided with a coating25 on the inside surface of electrode 18. Coating 25 is an air-filledmaterial such as air cell rubber. Because of coating 25, motion of thecone will not cause appreciable movement of any filling material orfluid which may be introduced inside the cone for the purposes ofhydraulic support and insulation. Air-filled material 26 is also appliedto the back side of the base 22 to isolate it from sound impinging onthe back side of the tranducer 11.

The transducer 11 can be included in a directional array assembly havingmany such transducer units mounted in a predetermined pattern relativeto a flat surface and all having their electrodes 14, 18 electricallyconnected. The'mechanical impedance of the transducer 11 can be designedto match the impedance of sound medium by a suitable adjustment of thegeometry of element 12 through a proper choice of the radius at thebase, the cone angle, and the wall thickness.

In Fig. 2 there is shown an omnidirectional unit comprising two coneelements 12 arranged base to base. The inner electrodes 18 of coneelements 12 are connected together at 27. Fig. 2 provides simple andcompact means for constructing an omnidirectional transducer, by usingtwo cones and insulating coatings and cabling provisions to produce atransducer. If the polarizations of the two cones are suitably chosen,the connections as shown are in series and the output voltage (also theelectrical impedance will be twice that of a single unit. When thedevice is very small as compared to a wavelength, a cone provides asupport of desirable mechanical properties for a similar cone. The sameconnections are assumed for Figs. 5 and 7.

In Fig. 3 transducer 11 is shown in outline With broken parallel lines31 to represent imaginary dividing planes for dividing the cone element12 into rings 32 parallel to the base 22. The ring 32 of largestdiameter, which is adjacent the base 22, possesses a particular radialvibration resonant frequency. The ring 33 next above possesses aresonant frequency slightly higher than that of ring 32. Likewise theother rings have correspondingly higher resonant frequencies up untilthe vertex 34 is reached Where resonance of maximum frequency can beexcited. This maximum resonant frequency can coincide with a resonantvibration involving the whole cone in a mode of motion which varies thewall thickness. Thus the transducer will possess a broadband basedresonance which is distributed between two well defined frequencylimits.

Fig. 4 shows a conical transducer element such as 12 in Fig. l in whichthe imaginary separations 31 of the zones as illustrated in Fig. 3 ismade real by incorporating circumferential grooves 35 to partiallydecouple the portions of the separate Zones. The conical construction ischaracterized by useful sensitivity at frequencies that are lower thanits lowest resonant frequency which is the resonant frequency of thering section adjacent the base. We have separate rings, each having itsown natural resonant frequency in radial oscillation. If all theincremental ring transducers are effectively in parallel, and each ishighly efficient at its resonant frequency, a band of relatively highefiiciency results, in principle, for the transducer as a whole.

In Fig. 5 is shown an omnidirectional air transducer similar to that ofFig. 2 but having a cushioning support 38 and a housing 39.Omnidirectional air transducers can also be constructed using thesensitive cone principle in other types of constructions than shown inFig. 5. In any case they do not include air-filled material such as 25in Fig. 1.

Figs. 6 and 7 illustrate two further modifications of the transducer foruse as underwater omnidirectional transducers. In Fig. 6 shielding 44 isincluded to shield against vibration from above; also the lower conicaltransducer is included in a plastic housing 47. Both conical transducersof Fig. 7 are included in plastic housing 47.

The modification shown in Fig. 8 illustrates the construction of anunderwater transducer embodying the features of Fig. 1 together with theaddition of a metal weight 45 afifixed in the vertex of the cone 12 forthe purpose of lowering and narrowing the resonant frequency band andfurther increasing efficiency. The use of such loading is advantageousfor sonic applications.

The weighted vertex, Fig. 8, alters the vibratory me chanics of thedevice, lowering the resonant frequency of the simple mode in which thevertex moves axially and the base moves radially. This can beadvantageous when narrow band (resonant) operation is desired, underwhich conditions improved efiiciency results. The important feature andproperty of the device of Fig. 8 is the weighted cone version as a lowfrequency receiver. The

and may be useful in this sense.

4 pliant support for the cone of Fig. 8 were mounted on the rigid innerwall of a hollow closed container, such as a sphere or a cylinder withspherical ends, the device would be sensitive to underwater sound, as anacceleration or displacement sensing device, inertia actuated. It wouldthus be directional, even when small compared to a wavelength (incontrast to pressure devices which must be large in wavelengths toachieve directionality). Hydrophones which are directional at lowfrequencies,

though small, are in demand.

The transducer of this invention is useful for directional andnondirectional broadband hydrophones and underwater projectors; withmass loading it is useful for semibroadband directional hydrophones andprojectors. It is generally useful in both air and water in a number offorms for listening, signaling, detection, vibration, pickup, two-waycommunication and the like.

In operation, the application of pressure to the exterior of the conewill cause radial expansion of the cone. Because of the volumeelectrical polarization of the material, a potential difference betweenthe thin metal electrodes coating the conical inner and outer surfacesof the cone element will appear. Due to the elements conical shape, itacts as a series of adjacent rings each of which has a differentresonant frequency thereby yielding a broadband frequency response. Byattaching a weight at the apex the band may be narrowed and theefficiency further increased.

Obviously many modifications and variations of the present invention arepossible in the light of the above teachings. It is therefore to beunderstood that within the scope of the appended claims the inventionmay be practiced otherwise than as specifically described.

I claim:

1. In a transducer, a sensitive element in the form of a hollow cone,said sensitive element being formed of electrostrictive material, a pairof electrodes one on the inner and one on the outer face of said cone, alayer of air-filled rubber covering the inside of said cone, and a ringupon which said sensitive element rests for supporting said sensitiveelement, said ring being formed of metal, said ring being provided withslots parallel to its axis to provide radial flexibility.

2. In a transducer, a sensitive element in the form of a hollow cone,said sensitive element being formed of electrostrictive material, a pairof electrodes one on the inner and one, on the outer face of said cone,a layer of airfilled rubber covering the inside of said cone, a ringupon which said sensitive element rests for supporting said sensitiveelement, said ring being formed of metal and provided with slotsparallel to its axis to provide radial flexibility, a base supportingsaid ring, and air-filled rubber material on the side of said baseopposite said cone.

References Cited in the file of this patent UNITED STATES PATENTS1,450,246 Cady Apr. 3, 1923 2,051,200 Christenson Aug. 18, 19362,452,085 Turner Oct. 26, 1948 2,487,962 Arndt Nov. 15, 1949 2,565,159Williams Aug. 21, 1951 2,638,577 Harris May 12, 1953

